A YIG resonator is a YIG film grown on a non-magnetic substrate such as gadolinium gallium garnet (GGG) and exposed to a biasing field. Such resonators are useful for microwave integrated circuit filters due to a high Q value resonance characteristic in the microwave frequency band, a compact planar structure, and suitability for mass production by selective patterning processes.
Epitaxially grown YIG films have low magnetic losses, thereby providing a good propagation medium for magnetostatic waves. Typically, a single YIG resonator is provided for each channel of a microwave channelizer to filter a narrow band signal from a wide band (or multi-channel) microwave input signal. More specifically, a microwave channelizer subdivides wide band signals in the range of 1 gigahertz and higher into a plurality of comparatively narrow band signals. Each channel filters the wide band signal to pass a particular narrow band signal. For magnetostatic waves (MSW) propagating in a YIG film of certain dimensions, the resonator dimensions and the bias magnetic field define a pass band center frequency of a respective channel.
In general, YIG resonators have applications for microwave filters, oscillators and communication systems.
Referring to FIG. 1, a typical YIG resonator 10 is shown in which a YIG film 12 of thickness d is epitaxially grown in a non-magnetic GGG substrate 14. Two microstrip lines 6,18 couple microwave energy into and out of the resonator 10 to provide respective input and output signal paths. The lines 16,18 are formed on a dielectric material 20 of height h. The YIG film 12 is disposed on the same material which separates the lines 16,18 and the film 12 from a metallic ground plane 22.
Magnetostatic waves typically are generated by passing current through a wire or conductor (such as lines 16 and 18) adjacent to the YIG film. The rf magnetic field surrounding the wire induces MSW propagation in the YIG film. The YIG film functions, in effect, as a waveguide.
A forward surface wave is propagated in the film 12 if a magnetic biasing field H is applied in the plane of the YIG film perpendicular to the direction of propagation. A forward volume wave is propagated if the magnetic biasing field H is applied normal to the plane of the YIG film 12. A backward volume wave is propagated if the magnetic biasing field H is in the plane of the YIG film in the direction of propagation. In FIG. 1, the magnetic biasing field H.sub.0 is applied normal to the plane of the YIG film 12 to propagate magnetostatic forward volume waves.
Conventionally, the resonator is tuned by altering the field strength of the magnetic biasing field and thereby altering the permeability of the YIG film and the center frequency of the resonator 10.
As shown in FIG. 2, a microwave channelizer 30 typically is formed of several resonators 32a, 32b, 32c, . . . forming a filter bank so that each resonator forms a separate channel for filtering a signal of particular frequency from a wide band microwave input signal S.sub.i. As practiced, each channel includes identically proportioned YIG films and the resonators or delay lines are subjected to a gradient of a magnetic biasing field (not shown). Thus, each resonator (or delay line) is exposed to a magnetic biasing field with a different intensity, and as a result has a different center frequency. The channelizer enables spectral analysis of a microwave input signal by receiving the input signal S.sub.i at each resonator and passing as the output signal S.sub.o from the resonator, only the narrow band of frequencies about the resonator's center frequency.
A problem with using a YIG resonator in certain applications is that the passband may be narrower than desireable.